TWO issues
1) the 8-pin connector, one of the pins might be damaged and not plugged in all the way
2) multi rail PSU can often cause problems, doesn't matter what your total AMPs are. you have 18x3 not 42. you might be overloading one of the rail, and not using fully one.

when you have single rail PSU, you never have to worry about overloading one rail, or which cables to use

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